Add rollover metadata containing all available keys

[tvdijen]

We sign our AuthnRequests to our IdPs, so we need to have metadata containing both the current and the rollover-certificate.
With this patch, the default metadata (keyslug=default or no keyslug) will contain all the configured keys.

[cdbesten]

Dit lijkt me zeer gewenst. Wij moeten volgend jaar op productie ook rollover'en en alles wat dit proces soepeler kan laten verlopen is mooi meegenomen.

[johanib]

@kayjoosten Can you:

• Add .feature test
• Check test coverage (unit needed/possible)
• Review implementation
• Assume current implementation is correct